Axially oriented pistoned variable displacement devices such as variable displacement motors are constructed with means for positioning the swash plate which determines displacement of the device. Such devices generally have a plurality of pistons reciprocably mounted in a circular arrangement of axially aligned cylinders in a cylinder block. The cylinder block is mounted for rotation on a drive shaft while each piston is operably connected to a pivotable swash plate angularly movable on an axis perpendicular to the drive shaft axis. Movement of the swash plate through such an angle varies the stroke length of the individual pistons as the cylinder block is rotated about the drive shaft axis. The swash plate, or a portion thereof, remains stationary relative to the drive shaft with the pistons and piston rods rotating with the cylinder block.
In such devices, the swash plate may be universally mounted about the drive shaft and fixed against rotation by interconnection with the device body. The mounting of the swash plate can include a pair of trunnions integrally formed with the swash plate and pivotally mounted in the device body. Positioning of the swash plate may be accomplished by a plurality of diametrically opposed and axially aligned actuating means in the form of hydraulic motors. The actuating means may be integrally formed with the device body and interconnected with the swash plate through the use of connecting rod means. The connecting rod means connecting the actuating means to the swash plate may be positioned outwardly of the cylinder axis a distance greater than the radius of the cylinder block in order to take advantage of the additional lever arm provided by such positioning, since the combination of hydraulic pressure and the lever arm must be sufficient to overcome the force acting against the actuating means by the pressure in the rotating cylinder block of the device itself. To utilize system pressure to position the swash plate requires a combination of greater effective cylinder area in the actuating means and/or lever arm as described above. However, by markedly increasing the cylinder area one can accomplish control of the swash plate positioning without utilization of a high pressure. To markedly increase the effective cylinder area requires either a larger device body or an "added on" actuating means.